1. Technical Field of the Invention
The present invention relates to a high frequency power device for supplying an electric power to a load such as a plasma processor for performing, for instance, a plasma etching and a plasma CVD.
2. Description of the Related Art
As a plasma processing system for processing (a plasma etching, a plasma CVD, etc.) an object to be processed such as a wafer, a liquid crystal substrate or the like by using a plasma generated by the use of a high frequency power, a plasma processing system using the high frequency power of different frequencies is known as shown in FIG. 6.
FIG. 6 is a block diagram showing the connecting relation of the plasma processing system using the high frequency power of the different frequencies.
In FIG. 6, a usual first high frequency power device 50 is a power device for supplying a first high frequency power (refer it to as a first high frequency, hereinafter) to a load 5 through a transmitting line 2, a first matching device 3 and a connecting part 4. An output frequency of the first high frequency power device 50 is designated by a first frequency f1 and a period of the first frequency is designated by t1. Further, a second high frequency power device 6 is a power device for supplying a second high frequency power (refer it to as a second high frequency, hereinafter) to the load 5 through a transmitting line 7, a matching device 8 and a connecting part 9 to the load 5. An output frequency of the second frequency power device 6 is designated by a second frequency f2 and the period of the second frequency is designated by t2. The first frequency is higher than the second frequency. For instance, as the first frequency, the frequency such as 13.56 MHz, 27.12 MHz, 40.68 MHz. etc. is employed. Further, as the second frequency, the frequency such as 400 kHz, 2 MHz etc. is used. In such a way, the high frequency power devices of this type ordinarily output the high frequency power of the frequency not lower than several hundred kHz.
The first high frequency outputted from the first high frequency power device 50 mainly serves to generate a plasma in the load 5. Further, the second high frequency outputted from the second high frequency power device 6 is used for a bias for efficiently performing a process (a plasma etching, plasma CVD, etc.) in the load 5. Two kinds of high frequencies outputted from the two high frequency power devices are superimposed and the superimposed high frequencies are applied to an electrode in the load 5.
To control the outputs of the high frequency power devices, a method for controlling forward wave powers respectively outputted therefrom to prescribed levels or a method for controlling a load side power obtained by subtracting a reflected wave power from the forward wave power to a prescribed level are employed.
Now, the high frequency power device 50, the matching device 3 and the load 5 will be mainly described below.
The matching device 3 is a device used for the purpose of matching impedance between the high frequency power device and the load 5 by matching a power source side impedance Zo (ordinarily, 50Ω) looking toward the high frequency power device 50 side from the input end 301 of the matching device 3 via the transmitting line 2 with a load side impedance ZL (the impedance of the matching device 3, the connecting part 4 to the load 5 and the load 5) looking toward the load 5 side from the input end of the matching device 3.
The matching device 3 includes a variable impedance element not shown in the drawing (for instance, a variable condenser, a variable inductor, etc.) therein and has a function for changing the impedance of the variable impedance element so as to match the impedance between the high frequency power device 50 and the load 5. More specifically, assuming that an impedance (an output impedance) looking toward the high frequency power device 50 side from an output end 501 of the high frequency power device 50 is designed to have, for instance, 50Ω, and the high frequency power device is connected to the input end of the impedance matching device 3 in the transmitting line 2 having the characteristic impedance of 50Ω, the impedance matching device 3 changes the impedance of the variable impedance element so as to convert the load side impedance ZL looking toward the load 5 side from the input end 301 of the impedance matching device 3 to 50Ω.
The load 5 is generally called a plasma processor and is a device having a chamber provided with electrodes therein to process or work (etching, CVD, or the like) an object to be processed such as a wafer or a liquid crystal substrate conveyed into the chamber. The load 5 introduces plasma discharging gas into the chamber to process or work the object to be processed and applies the high frequency power (voltage) supplied from the two high frequency power devices to the electrodes therein to generate a high frequency electric field between the electrodes, discharge the plasma discharging gas and obtain a plasma state. Then, this plasma is employed to process the object to be processed.
Now, the structure of the high frequency power device 50 will be described below.
FIG. 7 is a block diagram showing a structural example of an ordinary high frequency power device. An amplifying part 52 uses a DC power supplied from a DC power source part 51 to amplify an oscillating signal Vin outputted from an oscillating part 59 and output a high frequency power having an output frequency of a radio frequency band. The high frequency power amplified in the amplifying part 52 is supplied to the load 5 through a filter part 53 for mainly removing higher harmonics and a directional coupler 54. Further, a forward wave power value Pf is calculated in a forward wave power calculating part 55 in accordance with a forward wave voltage detected in the directional coupler 54. An output power control part 58 compares an output power setting value Pset of the high frequency power set in an output power setting part 57 with the forward wave power value Pf calculated in the forward wave power calculating part to control the output level of the oscillating signal Vin of the oscillating part so that both the values are equal to each other. That is, the output power control part controls the output level of the oscillating signal Vin of the oscillating part to control the output of the high frequency power to be constant. The high frequency power device may be constructed in such a way that a reflected wave power value Pr is calculated from a reflected wave voltage detected in the directional coupler 54 and a calculated load side power value obtained by subtracting the calculated reflected wave power value Pr from the calculated forward wave power value Pf is controlled to be constant. Such a high frequency power device is disclosed in JP-A-2003-143861.
FIG. 8 is an image diagram when the plasma is generated in the chamber of the load 5. As shown in FIG. 8, when the high frequency power (voltage) is applied to the electrodes in the chamber from the two high frequency power devices to generate the plasma, an electric conductive plasma and an insulating sheath are formed between the electrodes. In FIG. 8, a rectangular figure surrounding the periphery of the plasma is represented as an earth side electrode.
FIG. 9 shows an electric equivalent circuit of the load 5 when the plasma is generated in the chamber. As described above, since the plasma is electric conductive and the sheath has insulating characteristics, a loss part in the plasma can be expressed as a resistance and the sheath can be expressed as a condenser as shown in FIG. 9.
Now, a state will be described when a plurality (for instance, two) of high frequency power devices 50 and 6 having different output frequencies supply the high frequency power to one load 5 by using the electric equivalent circuit shown in FIG. 9.
Ordinarily, when the two high frequency power devices are used, a second high frequency (second frequency) for a bias is lower than a first high frequency (first frequency). In such a case, when there is a large difference between the output frequencies of the two high frequency power devices, the second high frequency causes a large reflected wave to be generated in the first high frequency power device 50 side.
Specifically, as shown in the equivalent circuit in FIG. 9, when the sheath is considered to be the condenser, the voltage across the condenser changes due to the influence of the high frequencies outputted from both the high frequency power devices 50 and 6. However, since the first frequency is higher than the second frequency, the voltage across the condenser changes as if the voltage were modulated by the second frequency. Consequently, the thickness of the sheath varies in the same period as that of the second frequency. When this phenomenon is considered in the equivalent circuit shown in FIG. 9, the phenomenon corresponds to the change of a distance between the electrodes of the condenser. Therefore, the capacity of the condenser changes.
Further, since the state of the plasma varies together with the change of the state of the sheath, the impedance of the load 5 changes as if the impedance were modulated by the second frequency. Accordingly, since a part of a forward wave outputted from the first high frequency power device 50 is reflected due to the influence of the modulation having the same period as that of the second frequency, the reflected wave is generated.
At this time, the first matching device 3 may conveniently match the impedance following the modulation of the second frequency. However, as described above, since the first matching device 3 drives the variable impedance element (for instance, the variable condenser, the variable inductor, etc.) to match the impedance, the first matching device cannot follow a high speed change such as the modulation of the second frequency. Thus, the reflected wave cannot be reduced. Therefore, the generated reflected wave returns to the first high frequency power device 50 side.
Further, since the reflected wave is generated owing to the phenomenon like a phenomenon that the first high frequency is modulated by the second frequency, the frequency components of the reflected wave include the first frequency as a main component and the second frequency as a spurious component mounted thereon. Accordingly, most of the frequency components of the reflected wave are occupied by the first frequency and the frequency in the neighborhood of the first frequency.
FIG. 10 shows one example of the simulation of the forward wave and the reflected wave detected in the output end of the first high frequency power device 50 having a higher output frequency when the two high frequency power devices having different output frequencies supply the high frequency power to the one load 5. The setting value of the forward wave power outputted from the first high frequency power device 50 is 3000 [W].
In FIG. 10, FIG. 10A shows the forward wave voltage in the output end of the first high frequency power device 50. FIG. 10B shows the reflected wave voltage in the output end of the first high frequency power device 50. FIG. 10C shows the frequency components in the vicinity of the first frequency of the frequency components of the forward wave voltage shown in FIG. 10A. FIG. 10D shows the frequency components in the vicinity of the first frequency of the frequency components of the reflected wave voltage shown in FIG. 10B.
As described above, when the plurality of high frequency power devices having the different output frequencies supply the high frequency power to the one load 5, for instance, the reflected wave voltage as shown in FIG. 10(b) is generated in the high frequency power device side having the high output frequency. The reflected wave power at this time is about 950 [W] in a certain model. This shows a very large rate as high as about 30% of the output power.
Further, at this time, the influence thereof is given to the forward wave so that the forward wave voltage as shown in FIG. 10A is obtained.
Further, in such a case, the frequency components in the vicinity of the first frequency of the forward wave voltage and the reflected wave voltage are respectively shown in FIG. 10C and FIG. 10D. That is, in the case of an example shown in FIG. 10, it is understood that the first frequency substantially occupies the frequency components of the forward wave voltage. On the other hand, it is understood, as described above, that the frequency components of the reflected wave voltage include the first frequency as a main component and the second frequency as spurious component mounted thereon. As a result, as shown in FIG. 10B, the reflected voltage varies in the period t2 of the second frequency.
In the high frequency power device, the filter part 53 is ordinarily provided in the output side as shown in FIG. 7. However, since the filter part 53 is a low-pass filter for removing higher harmonics component to the first frequency as the main component, the filter part 53 cannot remove the frequency components in the vicinity of the first frequency. Accordingly, the filter part cannot remove the components of the second frequency mounted on the first frequency of the main component as the spurious part.
Consequently, the generated reflected wave passes through the filter of the first high frequency power device 50 to enter the high frequency power device 50, so that the reflected wave gives an adverse effect to the amplifying element in the high frequency power device. Further, since the generated reflected wave power reaches about 30% as high as the output as described above, the reflected wave has a large influence and may sometimes break the amplifying element in the high frequency power device.
On the other hand, when viewed from the second high frequency power device 6, the reflected wave having the frequency components of the first frequency returns to the high frequency power device side. However, since the filter provided in the high frequency power device is a low-pass filter for removing the higher harmonics to the second frequency as the main component, the filter can remove the frequency components of the first frequency. Accordingly, the second high frequency power device 6 side hardly receives the influence of the first frequency outputted from the first high frequency power device 50.
As described above, when the plurality of high frequency power devices having the different output frequencies supply the high frequency power to the one load 5, a problem arises that the high frequency power device having the high output frequency receives an adverse effect in the amplifying element therein owing to the reflected wave by the influence of the high frequency power device having the low output frequency.